Demodulating apparatus



Dec. 11, 1923.

H. A. AFFEL DEMODULATING KPPARATUS Filed Dec. 31. 1920 anvcwfoz PatentedDec. 11, 1923.

UNITED STATES PATENT errce.

HEBHAN A. AFFEL, OF BROOKLYN, NEW YORK, ASSIGNOR '10 AMERICAN TELEPHONEAND TELEGRAPH COMPANY, A. CORPORATION 01? NEW YORK.

DEHODULA'IING APPARATUS.

Application filed December 81, 19%. Serial No. 484,226.

To all whomz't may concern:

Be it known that I, HERMAN A. Arrnn, residing at Brooklyn, in the countyof Kings and State of New York, have invented cer- 6 tain Improvementsin Demodulating Apparatus, of which the following is a specification.

This invention relates to signaling by means of carrier currents, andmore particularly to demodulating apparatus for use in connection withcarrier signaling systems.

It has heretofore been proposed to transmit the ringing signals over acarrier sys- 16 term by transmitting the unmodulated carrier frequencycom onent along with the modulated side ban and cause the ringin signalat the-distant station to be controlled by interruptions of theunmodulated carrier frequency component.

It is one of the objects of this invention to provide a demodulatingapparatus which will combine the functions of detecting the voicesignals from the modulated side band and which will also receive theringing signals produced by interrupting the carrier frequency.

Another object of the invention is to roduce a demodulating apparatus inw ich both the unmodulated carrier frequency component and the side bandfrequencies will be suppressed, so that they do not appear in the outputcircuit of the demodulator.

Other and further objects of the invention will be clear from thefollowing description when read in connection with the accompanyingdrawing, the figure of which is a slmplified circuit diagram of oneembodiment of the invention.

Referring to the figure two vacuum tube detecting devices 1 and 2 aredisclosed.

The tube 1 comprises a heated filament 3, a plate 4, and a grid 5, andsimilarly the tube 2 comprises a heated filament 6, a plate 7, and agrid 8. The grids 5 and 8 are connected to the secondary windings 9 and10 of the transformer 11, whose primar winding 12 is connected to aninto coming circuit 13, over which the unmodulated carrier frequency andthe modulated side band frequencies may be impressed upon thedemodulator. 7 are connected together in parallel as shown and a commonbranch 14 of the parallel plate circuits leads to the filaments 3 and 6.The common branch 14 includes a direct current relay 15, whose windingis shunted by a condenser 16 for by-passing the alternating currentcomponents appearmg in the plate circuits of the tubes, and the commonbranch 14 also includes the primary winding 17 of a transformer 18,whose secondary winding 19 is connected in the outgoing circuit 20, overwhich the detected voice currents may be transmitted. The armature 21 ofthe relay 15 controls a circuit 22 leading to any suitable ringing aparatus.

ssuming that a modulated voice band, together with an unmodulatedcarrier frequency component, is incomin from the circuit 13 and istransmitted tilirough the primary winding 12 of the transformer 11 tothe secondary windings 9 and 10, it will be observed that since thegrids of the two tubes are connected in series with respect to the inputoscillations, the oscillations ofcorresponding frequency appearing inthe plate circuits will also flow in series from plate to plate withoutflowing in the common circuit 14 and consequently the fundamentalcarrier frequency and the frequencies included in thelside band will notin themselves be transmitted to the outgoing circuit 20. The tubes willfunction. however, to detect the low frequency signaling currents inaccordance with which the side band was modulated, so that thelowfrequency currents will appear in the branch 14 and be transmitted tothe circuit 20. An increase in the steady plate current flowing throughthe winding of the relay 15 will also be produced, due to the detecteddirect current component, as long as the fundamental carrier frequency(is applied to the detector. As soon as the unmodulated carrierfrequency component is interrupted, however, the steady plate currentagain drops to normal and the armatures of the relay 15 falls off andopens the ringing circuit 22.

The general theory underlying a balanced circuit of this character isfully set forth The plates 4 and in British Patent No. 102,503 acceptedNovember 30th, 1917. While the theory as outlined in said British patentwas applied particularly to the case of a balanced form of modulator, itis equally applicable to a balanced form of demodulator, since the twodevices operate upon the same principle. As set forth in said Britishpatent, if a variable voltage represented by 'v be impressed on theinput side of a simple demodulator, 'such as an ordinary vacuum tubedemodulator, the resultant voltage vam'raeae riation V appearing on theoutput side'of the modulator may be expressed as follows:

If now a second demodulating tube be connected as proposed b the presentinvention, the input currents wi lbe applied in opposite phase relation,and, representing the output voltage variation by V and the inputvariation by v, the output voltage variation of the opposing tube may berepresented as follows:

In equations 1 and 2 the quantities a, 6, etc. are constants dependingupon the characterof the demodulating tubes. Let it now beassumed thatV" represents the voltage variation in a common or combined outputcircuit of the two tubes, whose output circuits are operating inopposite phase then V" will equal V-l-V. Hence, comblnmg equations 1 and2, and omitting the fourth power term which is negligible, we have:

It will now be assumed that the unmodulated carrier wave incoming'fromthe circuit 13 is of frequency This wave may then be mathematicallexpressed as equal to A sin Qt, wherein X represents the amplitude ofthe carrier-wave and t represents the time. The modulated side band ismade up of a number of frequencies corresponding to the frequencies inthe original modulatlng signal wave, but it will be sufiicient toconsider the case of a single side wave whose frequency may be assumedto be 5-;- This side wave which would be produced by modulating thecarrier wave. by a singing note ma be mathematically expressed as equalto sin (Pt), wherein B is the amplitude of the signal wave t inaccordance with the time, and a equals any arbitrary phase angle whichmay be assumed. From these two expressions it will be at once apparentthat the input voltage impressed upon the modulator may be representedas follows:

MA sin Qt+B sin (Pt- 1) Substituting this value of v in equation (3), wehave:

sponds to the sum of the carrier frequency and the side frequency. Thisfrequency is so high as to be above audibility, and if desired may beeliminated by tuning, and hence may be considered as taking no part inthe detecting operation. The fourth expression is negligible because Bis usually very small as compared with A. The first expression on theleft hand side of the equation, representing the square of theunmodulated carrier wave, may be written in expanded form, so thatequatlon 5-becomes:

Considering the first expression on the left hand side of the equation,it will be seen that it comprises a constant term 0A which representsthe direct current component resulting from the modulating action of thebalanced detector upon the unmodulated carrier frequency. This directcurrent component, it will be observed, appears as one of the factors ofthe squared term and the unmodulated carrier frequency itself and theside frequency do not appear in the output circuit. The remainingportion of the first term, which is cos Qt, is a term corresponding totwice the carrier frequency, and will also be above the audible limit,and may be eliminated by filters. The constant term A representing thedirect current component in flowing through the winding of the relay 15causes it to attract its armature so long as the carrier fre uency isapplied to the demodulator, but when the carrier frequency isinterrupted this component is no longer present and the armature of therelay is permitted to fall off and operate the in the following claims.

What is claimed is:

1. A demodulating apparatus comprising a pair of balanced detectingelements, the circuits of said detecting elements having a commonbranch, and means in said com.- mon branch responsive to the detectedlow fr uency signaling current and to the unm ulated carrier wave.

2. A demodulating apparatus comprising a pair of balanced detectingelements, the circuits of said elements having a common branch, means insaid common branch responsive to detected low frequency signalingcurrents, and means in said common branch responsive to the unmodulatedcar- I rier frequency.

3. A demodulating apparatus comprising a pair of detecting elements, thecircuits of said detecting elements having a common branch and saidelements being connected in such phase relations with respect to eachother as to suppress frequencies orresponding directly to theunmodulated carrier and the frequencies of the modulated side band,means in said common branch responsive to the low frequency currentdetected from the modulated side wave, and means in said common branchresponsive to the direct current component detected from the unmodulatedcarrier component.

4. In a demodulating apparatus a pair of detecting elements having inputand output circuits so connected that said input and out ut circuitshave branches common to hot elements and means in said common branch ofthe output circuits responsive to detected low frequency signalingcurrents and responsive to the unmodulated carrier frequency component.

5. A demodulating apparatus com rising a pair of detecting elements eachaving input and output circuits, the said input and output circuitsbeing so connected as to have branches common to both elements, means inthe common output branch responsive to detected low frequency signalingcurrents, and means in said common branch responsive to the unmodulatedsignal component.

6. A demodulating apparatus comprising a pair of detecting elements eachhaving input and output circuits, said input and output circuits beingso connected as to have branches common to both elements wherebyfrequencies correspondin directly to the unmodulated carrier and t emodulated side band will be suppressed from the output circuits, meansin the common branch of said output circuits responsive to the lowfrequency si aling currents detected from the side and, and means insaid common branch res onsive to the direct current component etectedfrom said unmodulated carrier.

7. A demodulating apparatus comprising a gair of vacuum tubes eachhaving grid an plate electrodes, a serial connection between said grideleetrodes and means whereby incoming unmodulated carrier components andmodulated side band frequencies are serially im ressed upon said grids,a series connection between said plates and a common branch soassociated with said plates that they will be in parallel with respectthereto whereby frequencies directly corresponding to the unmodulatedcarrier frequency and the modulated side band frequencies will besuppressed from said common branch, and means in said common branchresponsive to detected low frequency signaling currents and responsiveto said unmodulated carrier component.

8. A demodulating apparatus comprising a pair of vacuum tubes eachhaving grid an plate electrodes, a serial connection be tween said gridelectrodes and means whereby incoming unmodulated carrier components andmodulated side band frequencies are serially impressed upon said grids,a series connection between said plates and a common branch soassociated with said plates that they will be in parallel with respectthereto whereby frequencies directly.

corresponding to the unmodulated carrier frequency and the modulatedside band frequencies will be suppressed from said common branch, meansin said common branch responsive to detected low frequency signalingcurrents and means in said branch responsive to the unmodulated carriercomponent.

9. A demodulating apparatus comprising a pair of vacuum tubes eachhaving grid and plate electrodes, a serial connection between said gridelectrodes and means whereby incoming unmodulated carrier components andmodulated side band frequencies are serially impressed upon said grids,'a series connection between said plates and a common branch soassociated with said plates that they will be in parallel Q emme withrespect thereto whereby frequencies branch responsive to the directcurrent comdirectly correspondin. to the unmoduleted ponent detectedfrom said unmoduleted car- 10 carrier frequency en the modulated siderier component.

band frequencies will be suppressed from In testimony whereof, I havesigned said common branch, means in said commfy name to thisspecification this 30th day mon branch responsive to the low fre- 0December, 1920.

qpency signaling currents detected from said side band, and means insaid vcommon HERMAN A. AFFEL.

